The long-term objectives of this proposal are to reveal mechanisms operating at the cellular and microscopic level for events recorded during spontaneous impulse initiation and conduction observed at the macroscopic level in sinoatrial node and atrial maps. The studies are designed to reveal the basis for the multicentric or impulse origin and the link between changing patterns of complex atrial activation and changing heart rate in the SA-nodal preparation. Further, that conduction from the SA-node is non-uniform and impulses can only exit at a limited number of sites where there are nodal-atrial connections. The studies center about the simultaneous recordings of 250 extracellular potentials and intracellular pacemaker potentials from microelectrodes in an isolated perfused atrial preparation. Other studies will examine the morphology and cellular electrophysiology of pacemaker cells using whole-cell voltage-clamp techniques, focusing on a previously uncharacterized pacemaker cell with unique spider-like morphology and ion channel activity, which may be the dominant pacemaker cell type. An interesting aspect of these cells are the long dendritic-like projections which may serve an intercellular regulatory and communication function, affecting cell group behavior. In addition, the pathway of impulses from the earliest intracellular pacemaker region into the atrium will be determined by correlating electrophysiologic data with three dimensional reconstructions of the sinus node and surrounding atrial tissue at selected areas of functional importance. Other studies will focus on the difference in connexin protein subtypes associated with interconnections among the atrial pacemaker complex (APMC) cells. Human studies will involve in vitro perfused atria in which intracellular, whole-cell and extracellular potentials will be obtained and correlated with morphologic data and studies of isolated SA-node pacemaker cells.